Many proposals have been made and known for a process for the polymerization of olefins in the presence of a catalyst formed by a solid catalyst component comprising a titanium halide compound, a magnesium compound and an electron donor compound as essential components, an organic aluminum compound and a third component such as silicon compound. Further, a process by which a crystalline polyolefin having a high stereoregularity can be obtained in the presence of such a catalyst in a high yield has been extensively studied.
For example, JP-A-63-3010 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-1-221405, JP-A-1-315406, JP-A-3-227309, JP-A-3-70711, and JP-A-4-8709 disclose a process for the production of a polymer having a high stereoregularity in a high yield in the presence of an olefin polymerization catalyst formed by a solid catalyst component prepared from dialkoxymagnesium and titanium tetrachloride as main starting materials, an organic aluminum compound and a third component such as silicon compound.
Further, various proposals have been made for an olefin polymerization catalyst formed by (a) a solid catalyst component comprising a halogenated aluminum compound, a magnesium compound and a halogenated titanium compound as essential constituents, (b) an organic aluminum compound and (c) a third component such as organic acid ester and silicon compound. For example, JP-A-55-161807 proposes a catalyst formed by magnesium chloride, halogenated titanium, an organic acid ester, a halogenated hydrocarbon compound, and a halogenated aluminum compound. JP-A-61-31402 discloses a process for the preparation of a polymer having a high stereoregularity in a high yield in the presence of a catalyst prepared from (a) a solid catalyst component obtained by a process which comprises reacting (i) a reaction product of a halogenated aluminum compound with a silicon compound with (ii) a magnesium compound, and then reacting the reaction product with a halogenated titanium compound and a phthalic acid ester, (b) an organic aluminum compound and (c) a silicon compound.
The foregoing various techniques focus on the development of a catalyst component which is active enough to allow the omission of a so-called deashing step, i.e., step of removing catalyst residues such as chlorine and titanium remaining in the polymer produced by the polymerization of propylene in the presence of a catalyst as well as on the enhancement of the yield of a stereoregular polymer or the durability of the polymerization activity. These techniques can provide excellent results on these purposes.
Referring to propylene polymer, on the other hand, for example, JP-A-7-25946 discloses a propylene polymer having a high crystallinity of boiling n-heptane-insoluble component, a high stereoregularity and an extremely long meso run-length.
The propylene polymer obtained by the foregoing conventional techniques exhibits a high heat deformation temperature, a high melting point and a high crystallization temperature and thus exerts considerably good effects in respect to rigidity and heat resistance. Thus, the propylene polymer obtained by the foregoing conventional techniques can be utilized for various molding uses such as extrusion molding into sheet, film or the like, blow molding and injection molding. However, some problems have been still left unsolved in the molding of polypropylene. In particular, if the foregoing polymer having a high rigidity is subjected to extrusion molding into sheet, film or the like, some troubles can occur such as breaking during high speed molding and loss of transparency of the resulting molded product.
As a method for solving these problems there has been commonly known a method which comprises lowering the stereo-regularity of the propylene polymer and hence the energy required to work the propylene polymer. However, this solution is disadvantageous in that the quality of the resulting product can be rather deteriorated than improved because the propylene polymer contains a large amount of atactic propylene polymers. Examples of other solutions which have been attempted include a method which comprises allowing a small amount of ethylene in the polymerization system as a comonomer. Although this method makes it possible to control somewhat the crystallinity or density the resulting polymer, this method disadvantageously results in the complication of production process and the rise in the product cost and causes an undesirable phenomenon, i.e., rise in the percent occurrence of atactic polypropylene having an extremely low stereoregularity.
Moreover, in order to improve the transparency of propylene polymer, the addition of various nucleating agents to the propylene polymer produced is attempted in JP-A-2-265905 and JP-A-2-29444. However, this solution is disadvantageous in that the addition of nucleating agents causes the generation of odor during working. Further, since the nucleating agents thus added are insufficiently dispersed in the polymer, this solution leaves something to be desired in the improvement of transparency.
As mentioned above, these conventional techniques leave something to be desired in solution to the foregoing problems. It has thus been keenly desired to further develop a propylene homopolymer having a high stereoregularity which can be fairly worked into a sheet or film having an excellent quality.